In principle any measurement, or set of measurements, which allows the spatial distribution of a field to be determined can be considered an `imaging' technique. In that sense, any of the above-mentioned sensors can be used for imaging if repeated measurements can be made at different spatial positions over a time period where the field of interest does not change. For example, measurements using Rydberg atoms can achieve sub-wavelength imaging of microwave electric fields by moving, or rastering, the sensor with respect to the measured field [1]}. Furthermore, atomic vapours can also be used in imaging modalities where the full field is captured in a single shot. For example, room temperature alkali vapour can be used to measure the magnetic component of microwave fields by detecting the population transfer between atomic hyperfine ground states [2]}. Using this technique the field above microwave circuits has been imaged in a single shot at frame rates of around 10 Hz and spatial resolution below 100 micrometers [3]}.
Some other atom-based imaging techniques employ standard optical methods in combination with atomic-vapour line filters to provide narrow-band transmission and strong out-of-band rejection for imaging at specific wavelengths [4]} thereby allowing the signal of interest to be extracted from unwanted background light. Atomic line filters have been used for imaging applications ranging from astronomical solar monitoring [5]} and imaging of jet plumes [6]} to microscopy for single DNA molecule detection [7]}.
